This invention relates to a speed control system for a hydraulic elevator of the type which comprises a hydraulic jack including a vertically extending cylinder and a plunger vertically movable therein, a hydraulic system including a hydraulic pump for applying hydraulic pressure to the cylinder, and operative to control the hydraulic pressure applied to the cylinder for controlling the vertical movement of the plunger in the cylinder, and an elevator cage operatively connected to the plunger so that it is moved upward or downward with the vertical movement of the plunger. In one of the well-known speed control systems for such a type of hydraulic elevator, the speed control is carried out in such a manner that for upward travel, a part of the oil discharged by the hydraulic pump is bled off in a controlled manner to regulate the amount of oil supplied to the cylinder, while for down travel, the hydraulic pump is deactivated and the amount of oil discharged from the hydraulic cylinder through a return pipe by the weight of the elevator cage and plunger is regulated by a flow control valve provided in the return pipe. Such a speed control system is disclosed, for example, in U.S. Pat. No. 3,955,649 issued May 11, 1976 to Mitsuaki Takenoshita et al. and entitled "Device for Correcting Floor Level of Hydraulic Elevator" and U.S. Pat. No. 3,892,292 issued July 1, 1975 to Mitsuaki Takenoshita et al. and entitled "Hydraulic Elevators."
The speed control system of this type has disadvantages in that pressure loss in the flow control valve is converted into heat energy, thereby increasing the temperature of the oil in the hydraulic system. The temperature of oil in the hydraulic system is generally required to be held at or below 60.degree. C. At higher temperatures than this, oil viscosity is reduced, the flow rate of the oil flowing in the pipe changes, thus changing the moving speed of the plunger of the hydraulic jack, with the result that the running speed of the elevator cage is changed, thereby making difficult accurate landing of the cage at the desired floor level. This necessitates a large-sized cooler for preventing the temperature of the oil from increasing higher than a predetermined level. Provision of a large-sized cooler increases the cost of installation and maintenance. Further, in the cast of the cooler being of the air-cooled type, noises and required space are increased, and in the case of the water-cooled type of cooler, limitation of water use in dry seasons may prevent the operation of the hydraulic elevator.
To obviate such disadvantages, a recently suggested speed control system comprises two types of hydraulic pumps, one for high speed and the other for low speed, without any flow rate control valve. The use of two hydraulic pumps and motors, however, undesirably increases the space occupied by a power unit constituting a hydraulic pressure source on the one hand and the installation cost on the other hand. Also, the fact that the oil flows discharged from the two hydraulic pumps are used in combination for the purpose of speed control results in the disadvantageous necessity of complicated regulation at acceleration or deceleration.